The water / cementitious (w/c) ratio is used in both tensile and compressive strength analyses of Portland concrete cement. This ratio is found from
w / c = w m / w c
where w m = weight of mixing water in batch, lb (kg); and w c = weight of cementitious materials in batch, lb (kg).
The ACI Code lists the typical relationship between the w/c ratio by weight and the compressive strength of concrete. Ratios for non-air-entrained concrete vary between 0.41 for a 28-day compressive strength of 6000 lb/in 2 (41 MPa) and 0.82 for 2000 lb/in 2 (14 MPa). Air-entrained concrete w/c ratios vary from 0.40 to 0.74 for 5000 lb/in 2 (34 MPa) and 2000 lb/in 2 (14 MPa) compressive strength, respectively. Be certain to refer to the ACI Code for the appropriate w/c value when preparing designs or concrete analyses.
Further, the ACI Code also lists maximum w/c ratios when strength data are not available. Absolute w/c ratios by weight vary from 0.67 to 0.38 for non-air-entrained concrete and from 0.54 to 0.35 for air-entrained concrete. These values are for a specified 28-day compressive strength f ’ c in lb/in 2 or MPa, of 2500 lb/in 2 (17 MPa) to 5000 lb/in 2 (34 MPa). Again, refer to the ACI Code before making any design or construction decisions.
Maximum w/c ratios for a variety of construction conditions are also listed in the ACI Code. Construction conditions include concrete protected from exposure to freezing and thawing; concrete intended to be watertight; and concrete exposed to deicing salts, brackish water, seawater, etc
